The natural gamma-ray API formation maintained by the University of Houston (“UH formation”) defines the API unit to which natural gamma-ray tools are calibrated. Unfortunately, the narrow borehole of the UH formation cannot accommodate logging-while-drilling (LWD) tools, and planned expansion of the university will soon make the formation unavailable. This paper lays the groundwork for replacing the UH formation with a combination of computer modeling and a single calibration point. The effectiveness of the method is illustrated with a wireline tool and an LWD tool.

This method defines a formation to be used with computer models (“digital API formation”) that emulates the UH formation. However, unlike the UH formation, the digital API formation has an uncased borehole. With modeling, it is easy to vary the size of the borehole to match the tool size being calibrated. To account for imperfections in the tool model, the model is calibrated by comparing its predictions to physical-tool measurements in a large tank of potassium chloride brine. Tool sensitivity is computed by dividing the calibrated count rate computed for the digital API formation by the API value assigned to the formation.

Designing the digital API formation began by developing a computer model that emulates the UH formation. The count rates computed with this model for a wireline tool matched the corresponding measured count rate to within 1%. Like the UH formation, the source of the digital API formation contains a combination of potassium, thorium, and uranium. The relative combination of these elements was determined so that the spectrum of photons on the surface of a wireline detector in the digital API formation is the same as in the UH formation. The absolute concentrations were defined so that a wireline tool would have the same count rate in both formations.

This method is shown to match the sensitivity of a wireline tool that was calibrated in the UH formation to within 1%. The method was also used to compute the sensitivity of an LWD tool, which compares favorably to the measured sensitivity determined with granite blocks. Using this method, the UH formation can be safely discarded. In addition, better agreement between wireline and LWD logs can be obtained because they are all calibrated in the same formation and in their natural logging positions. Details of the digital API formation are disclosed.